Microwave dryer equipment

ABSTRACT

Water is removed from ceramic pieces in a pre-drying stage of a microwave drying process in which microwave energy is absorbed more in thicker dielectric sections than in thinner sections. A thin dielectric partition provides a second lower cavity in a main microwave chamber. Microwave energy heats the piece rapidly to drive out the water initially to saturate the chamber and maintains the heat to condense water vapor on cooler surfaces. A homogeneous humidity distribution is achieved which provides fast and uniform drying.

United States Patent 1 1 m1 mamas Hallier et al. 1 May 1, 1973 [54] MICROWAVE DRYER EQUIPMENT Primary ExaminerJ. V. Truhe [75] Inventors: Bernard Lucien D. Hallier, Saint- Assistant Exammer Hugh Jaeger Attorney-C. Cornell Remsen, Jr. et al. Maur, La Varenne; Jean Louis Coquet, 87 Saint-Leonard de Noblat, both of France [73] Assignee: International Standard Electric Cor- 57 ABSTRACT Water is removed from ceramic pieces in a pre-drying stage of a microwave drying process in which 1 New York, microwave energy is absorbed more in thicker dielec- [22] Filed: Oct. 18, 1971 tric sections than in thinner sections. A thin dielectric partition provides a second lower cavity in a mam [21] Appl. No.: 189,866 microwave chamber. Microwave energy heats the piece rapidly to drive out the water initially to saturate the chamber and maintains the heat to condense water [52] US. Cl ..2l9/10.55, 34/1 vapor on cooler surfaces A homogeneous humidity [51 Int. Cl. ..H05b 9/06 distribution is achieved which provides fast and [58] Field of Search "219/1055; 34/1 if d i 56] References Cited 3 Claims, 4 Drawing Figures UNITED STATES PATENTS 3,276,138 10/1966 Fritz ..34/l

Patented May 1, 1973 3,731,036

2 Sheds-Sheet 1 F .4 Q24 Z4 ZJ 1J4 3 [nvenlors BERNARD L. D. HALL/6R JEAN L, COQUET B MW Attorney Patented May 1,1973

BERNARD L, D. HALL/5R JEAN. L. COQUET Allornq MICROWAVE DRYER EQUIPMENT BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a microwave heating process and equipment for predrying ceramic pastes used for manufacturing porcelain pieces, such as plates and dishes which are shaped on a boss. The microwave energy source may be tubes of the magnetron or klys- 1 tron types operating in certain frequency ranges devoted to microwave heating, for example 2,450 MHz.

2. Description of the Prior Art In the process of manufacturing table porcelain 1 pieces, there is one particularly important stage; that of predrying the plasticpaste. This is an operation which formerly required a great amount of time. In modern manufacturing, with high piece flows, the use of old processes for the predrying stage would create a bottleneck.

In present techniques, the material is first shaped by pressing and forming the paste on a plaster mold having the pattern of the final piece and by using a special molding machine such as known by the name of roller. It is then necessary to dry the paste. During the first drying phase, water leaks out at the same time that the piece is subject to shrinkage. After a certain quantity of water has been eliminated, the paste continues to dry, but water leakage is no longer accompanied by shrinkage.

That first predrying phase is a critical step since it is necessary to prevent any accidents such as cracks or deformations from occurring during that time.

In common methods, predrying is performed in free air, with paste water being eliminated by absorption into the mold pores as well as by slow evaporation. The predrying operation is over when the piece can be taken out of the mold without effort and is rigid enough to be set on a suitable form for complete drying (whitened drying).

Predrying was a time consuming manufacturing phase which required many handlings, large warehouses with numerous shelves, and a large stock of molds which had to be renewed rapidly to avoid dirt and plaster decomposition due to absorbed humidity.

Porcelain makers have sought to reduce predrying duration by placing molds within a tunnel and subjecting them to dry hot air flow and infrared radition. However, water evaporation is a surface phenomenon and, in that case, a superficial crust is formed which prevents the water inside the paste from leaking out. Shrinkage is not proper and cracks and deformations occur.

Predrying process analysis shows that in order to avoid cracks and deformations which may result from internal stresses and contractions, it is necessary that every piece part, whatever is its position or thickness, be subject to uniformly distributed humidity extraction. Otherwise the humidity rate per volume unit of the piece, which is substantially homogeneous after shaping, remains the same during the complete predrying.

A survey of existing predrying processes shows that most of them do not simultaneously satisfy the two required conditions: speed of operation and homogeneous humidity distribution.

For example, as already mentioned, the usual predrying process in free air without ventilation requires considerable time, about 12-24 hours, but this does not facilitate humidity uniformization.

A more rapid modern predrying process uses a tunnel with dry hot air ventilation. However, humidity extraction is obtained by progressive evaporation from the inside to the outside of the piece and, to a lesser degree, by absorption into the plaster mold pores. It is therefore difficult to obtain homogeneous humidity rate distribution inside the paste during the entire predrying operation.

The more recent process using microwave energy absorption due to dielectric losses in intersticial water inside the handled piece makes it possible in most cases to satisfy the two above mentioned conditions.

The earlier U.S. Pat. application Ser. No. 105,391, filed Jan. 11, 1971, and assigned to the same assignee as the instant case, describes a predrying process which selectively heats intersticial water by means of microwave radiation while dry air ventilation provides evaporation of water from the surface.

I If the microwave energy distribution is homogeneous, the whole piece rapidly reaches a certain temperature. Intersticial water is transferred to the surface where it evaporates due to the ventilation. By suitably adjusting microwave energy and dry air flow characteristics, it is possible, in most cases, to ensure uniformly distributed humidity evacuation. Experience confirms that rather simple shaped pieces and particularly pieces having regular thickness are handled very satisfactorily by the process.

Such a process is not as successful when the pieces have less simple shapes. This is particularly so for plates and dishes whose thickness in the base support area is thicker than in the wing or bottomareas. This failure may result from the fact that water migration speed toward the piece surface depends on the temperature and on ventilating air characteristics. Thicker regions are thus evacuated more slowly since water transfer time is roughly proportional to thickness.

In very old techniques such as in drying textiles in free air, it has often been observed that the thicker parts of woven material remain more humid then less thick ones.

SUMMARY OF THE INVENTION A purpose of this invention is to provide a process for ceramic pieces using a microwave energy source and to provide humidity evacuation conditions to ensure homogeneous humidity distribution inside handled piece during the entire operation.

According to the invention, during the major part of the operation, the piece to be predried is surrounded by its own humidity which is extracted in the form of vapor by microwave heating.

The devices used to supply microwaves in the present process are known and described in the above cited Patent Application.

The pieces to be dried are placed in an enclosure with metallic walls and are subjected to radiation from a source of microwave energy. It is known that the major characteristics of microwave heating is its selectivity. Only elements located inside enclosure which present high dielectric losses, such as the ceramic pieces, absorb much microwave energy and are heated. On the contrary, metallic walls and supports or walls of low loss dielectric material reflect and/or transmit energy with very little absorption. Therefore they are not heated if they thermally are insulated.

In particular, a dielectric plate of relatively small thickness and of low relative permittivity, will hardly change the electromagnetic field distribution and absorption conditions of the piece when placed inside an enclosure provided with microwave radiation.

According to one feature of the invention, the main microwave radiation enclosure includes a plate made of dielectric material having a relatively low permittivity K and low losses. The plate is located between the microwave energy source and the pieces being predried. Thus the dielectric plate defines a secondary enclosure in the main enclosure which is of good fluid tightness and which limits the volume for evaporated water.

According to another feature of the invention, the dimensions of the secondary enclosure or cavity having a volume V, including the dielectric plate position and the applied microwave power, are determined so that during a first predrying phase the piece temperature increases up to the temperature it, which corresponds to the water vapor saturation of the volume V, and so that during a second phase the temperature is held at the temperature If, while evacuated water vapor progressively condenses on the cooler surfaces located inside the said secondary enclosure.

According to a further feature of the invention, the microwave power applied during the first predrying phase is higher than the power applied during the second phase in order to reduce the temperature rise time up to t, and to increase the duration when saturated vapor is surrounding the pieces.

According to a still further feature of the present invention, the dielectric plate position is adjustable, even during predrying, by means of rods or flexible ropes acting as supporting means in a dielectric material of relative permittivity K,. The ropes slide in metallic tubes having an inside diameter 2r located out of the main enclosure, the product'2r V-K, being substantially lower than microwave radiation wave length.

It will be noted that in the present process there is no air forced ventilation. The only energy used is the microwave energy which causes the paste temperature to increase rapidly without increasing the temperature of main enclosure walls, the dielectric plate, or various dielectric supports and the plaster molds (to the extent that they are dried between two successive operations).

The water evaporated at the surface of the pieces under predrying begins to saturate the air in the secondary enclosure, then beyond the saturation point, condenses, on very dry areas of the pieces, on cold secondary enclosure walls, on the free mold surfaces and on the dielectric plate.

The saturating vapor and liquid water which have accumulated in the enclosure at the end of the predrying operation are evacuated by any suitable means such as suitably located drains, or sweeping by dry air ventilation, dry air being only introduced after the end of the predrying operation.

Due to the process of the invention, a balance is established between saturating vapor close to predried surfaces and humidity in the pieces so that drier parts have a tendency to reabsorb humidity while more humid parts will continue to lose it. Residual humidity inside the pieces then reaches a homogeneous distribution rate.

Other features of the present invention will appear more clearly from the following detailed description of aspeciflc embodiment of the invention in conjunction 0 with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING DESCRIPTION or THE PREFERRED EMBODIMENTS FIG. 1 shows a cross-section of the porcelain plate 1 still in a paste condition. It is placed with the inside downward on the plaster mold 2 on which the plate is shaped by forming and pressing by means of a special machine known as a roller" by ceramists. The mixing agent which is used for assembling dry ceramic powder grains is constituted by water. The water proportion necessary to be added into the dry powder is about 25 percent of the powder weight. The predrying operation must provide a piece rigid enough to be separated from the support mold without effort and must evacuate about 16 percent of the initial water.

By way of example, for a plate which in a paste condition has a whole mass of 600 g, initial water represents 120 g. The predrying operationshould provide evacuation of about 20 g per plate. I

During predrying, the predried piece is subject to shrinkage. Thus the free parts are subject to important local stresses while parts in close contact with the mold are prevented from shrinkage without mechanical effort.

Examination of FIG. 1 shows the various mechanical stresses endured by the piece in different parts.

Concerning the wings 3, paste progressively separates from the mold without effort along aa.

In the base support area 4, due to shrinkage, the piece grips the mold and cannot be shrunk except by rocking and forced sliding from point b to point b'.

On the bottom 5, such rocking is accompanied by torsion which results from at least three coexistent phenomena:

restraint of the piece at bb' which precludes normal shrinkage of the plate bottom;

a tight fit of the piece on the mold at bb' which makes it difficult for air to enter between the mold and the plate bottom and thus can produce a suctiongripping effect; and

the paste weight, which opposes the movement of the plate along the mold.

In all the presently existing predrying processes,

these phenomena occur. Their effects are much less noticeable when using the usual slow processes than in the more modern processes. These effects are minimized if, during the entire operation, the humidity rate is evenly distributed to the whole of the predried piece. But this is not so in processes which have a constant rate of water extraction from the inside to the surface. The thicker portions, such as 4 will thus dry more slowly than the less thick sections such as the wings 5.

The present process makes it possible to obtain an homogeneous distribution of humidity rate within the piece or the pieces being predried.

The FIG. 2 schematically shows an example of predrying equipment which permits implementation of the novel process.

This equipment operates in a step-by-step mode usable in a production line for porcelain plates.

The mechanical assembly includes a metallic frame 6. The lower part (in has transfer means including a motor 7 to control a driving device 8 and a conveying belt 9 of stainless steel which passes around pulleys l0 and 10'. The upper part 6b of the frame supports elevating means such as a hydraulic jack 11a fastened to an assembly of girders 11b permitting uniform distribu'tion of pressure forces, the jack being controlled by an hydraulic power source 12.

Part 6b also holds the magnetron power supplies 13 and a programmed control box 14 which provides all control signals for the equipment.

Inside upper frame 6b, supported by 11b, is a parallelepiped-shaped main microwave enclosure 15.Enclosure 15 is generally made of duraluminum or any other good conductor material. The enclosure has, for giving a specific example, a height of between five and ten times that of handled items including the plate and mold. Other dimensions depend on how many plates are predried in a batch. On the roof of the main enclosure 15 are the microwave sources 16 each including a magnetron and associated microwave circuit which is coupled to the interior of the enclosure 15 by slots.

The enclosure 15 and the items carried thereon may be elevated by the jack 11a and the girder assembly 11b. Each power supply 13 is linked by flexible cables 17 to the magnetron of a source 16.

Inside the enclosure 15, is a relatively thin plate 18 of insulating material having a low permittivity K and a small microwave dielectric loss angle. The plate 18 is supported parallel to the roof of 15, by the brackets 19, also of insulating material and which may be placed at various heights. With respect to the various fluids, the plate substantially insulates the upper part of 15 from the lower part, vapor diffusion being relatively slow between the edges of 18 and the vertical walls of 15.

During periods of microwave radiation, the enclosure 15 is lowered and is brought down into contact with the metallic belt 9 via a metallic braid 20 welded to the edges of the open face of 15. The pressure of the enclosure 15 on the belt 9 is controlled by the jack 11a and may be measured, for example, by a dynanometer, not shown.

Thus, the enclosure 15 and the belt 9 define the volume of a main cavity which contains the electromagnetic fields from the radiating microwave sources 16. The enclosure includes a first upper cavity 21 and the lower part of 15, the metallic belt 9'and the plate 18 also define the inside of a secondary cavity 22 which is substantially tight for water vapor that is evacuated from ceramic pieces being predried on the belt 9.

In order to obtain a good distribution of the microwave energy and to avoid standing wave phenomena inside the predried pieces:

the cavity 21 has linear dimensions substantially larger than the microwave length k;

there are a plurality of sources '16 which are independent so that microwave fields are combined within 21 with inconsistent phases;

each source 16 has an associated field stirrer located at the ceiling of 15 close to the slots which couple 16 to 21.

The plate 18, due to its thinness and relatively low permittivity K, has little effect on the electromagnetic field distribution; if e is the plate thinness, it is equivalent to the cavity height being increased by The items for carrying the predriedplate batch an d their molds are, for example, trays made of plastic material with low dielectric losses (such as polypropylene).

In a production line, while a batch n of plates and molds is positioned in cavity 15, lowered on the belt 9 and subjected to microwave heating, the batch n+1 from the roller machine is placed on belt 9 at the left of 15, while the plates of the batch n-1 on the right of 15, suitably predried, are separated from their molds by any convenient means and then handled in whitening drying ovens which may be usual microwave ovens.

When the handling of batch n is completed, the cavity 15 is elevated by the jack 11a after having stopped the operation of microwave sources 16. A suitable ventilating device sweeps out the vapor saturated air which had been accumulated in the enclosure 15 and also any water condensed on the walls. The belt 9 then moves forward to the right so as to place the batch n+1 under 15. The n goes out to the right ready to be taken out of the molds. Then, the cavity 15 is lowered and, after being certain that the pressure of the metallic braid 20 on the belt 9 is satisfactory, the microwave sources 16 are switched on and now cause the batch n+1 to be radiated. The following operations continue as mentioned above.

The control of each operation, depending upon its nature or duration, is performed by control box 14.

A predrying process according to the invention will now be described by using the equipments and devices of the FIG. 2. This description will emphasize certain quantitative elements which will show features and advantages of the invention.

To be specific, it will be assumed that it is intended to predry 300 plates per hour. 7

Pieces coming out of a ceramic paste shaping machine have a mass of 600 g including 480 g of dry material and I20 g of water.

Predrying is completed when piece is rigid enough to be separated without effort from the supporting mold. That corresponds to an evacuation of 20 g of water per piece. The plaster mold has a mass of about 1.5 kg.

avoided, such transformations being dangerous to the mold plaster contacting the piece. To be specific, X may be equal to 55C. Whatever rapid predrying process is used, a plate needs, as a minimum, the energy sufficient to increase the temperature of 120 g of water by 30C and the energy sufficient to vaporize 20 g of water, or, from simple calculations, 63 kilojoules.

Predrying 300 plates will accordingly need a minimum of 18,900 kilojoules, or 5.2 kWh.

Solid components of the ceramic paste are also heated by 30C; as their specific heat is about two tenths of that of water, the additional energy to be provided per plate is 12 kilojoules, or for 300 plates 3,600 Kilojoules or I kWh.

In most of the known processes, mold plaster and numerous supporting means are also heated which increases the energy to be provided. There is an advantage of predrying or drying ovens using microwave heating due to its selectivity. Mainly, to the extent that molds absorb very little quantity of water during the operation, dielectric losses in dry plaster are very low and molds are not substantially heated.

As a result, when microwave heating is used, the

energy required for predrying 300 plates per hour is about 6 kWh comprising 2 kWh for heating the paste and 4 kWh for vaporizing 20 g of water per plate (i.e. 6 kg for 300 plates).

In the process described in the above Application paste heating energy is provided mainly by microwave sources while vaporization energy is provided mainly by strongly ventilated dry air.

In this process, allthe energy needed for predrying is brought by the microwave sources since there is no ventilation.

By assuming that during each predrying operation, the plate batch is subjected to microwave radiation for 90 seconds in themain cavity 21 and that 30 seconds per batch are needed for various other steps, the power to be provided by microwave sources 16 is equivalent to (6X 4/3)=8 kW.

The number of batches per hour is thirty and each batch comprises 10 plates.

Minimum sizes of the enclosure are determined by the length needed on the belt 9 by ten plates and their molds and by the requirement for a microwave energy distribution as homogeneous as possible which is better'with a larger main cavity 21.

sizes as follows: length 1.5 m, width 1 m, height l.20 m, i.e. a volume of 1.8 m.

The necessary power of 8 kWh may be provided by four sources 16 each using a magnetron having a nominal power of 2.5 kW, the sources being suitably distributed on the roof of the main enclosure 15.

The positioning of the insulator material plate 18 will now be described, this plate'defining the secondary cavity 22 in main cavity 15 which is tight enough to limit the volume of either dry or saturated evaporated water.

It is known that when there is a balanced condition between a liquid and its saturated vapor at a temperature t,, the saturating vapor pressure p, and the specific mass M, of the saturated vapor are only dependent upon the temperature 1,.

The curves of the FIG. 3 show p, (bars) and M, (g/m") versus 1,.

It is assumed however that, for the case of the equipment shown in the FIG. 2, during each processing of a plate batch, the whole secondary cavity 22 and its contents are heated up to the selected temperature of 55C. The evacuated water quantity of 200 g, assuming that it is only saturating vapor, must then occupy a volume of4 m according to the curves of the FIG. 3.

It is to be noted that this volume is very important and requires large dimensions for the main cavity, substantially larger than for a normal microwave heating (1.8 m). The vapor only becomes saturated at the end of the operation and the homogeneous humidity dis tribution, expected from the process, has no time to be realized.

The whole secondary cavity 22 and its contents cannot be heated to 55C by microwave heating only because of selectivity of this type of heating and also because of the low conductivity of the present elements.

In practice, the saturating vapor for the selected tem-.

perature (55C) occurs when the evacuated water quantity in vapor form saturates the secondary cavity. The volume of the secondary cavity is roughly defined by the height of the plate 18 over the belt 9.

By way of example, the plate will be assumed at midheight of the main cavity which provides a volume for the secondary cavity of 0.9 m, the main enclosure having a volume of 1.8 m

According to the curves of the FIG. 3, when the saturating vapor condition occurs, the evacuated water quantity in the secondary cavity is g.

From this point, if heat is applied to the paste by microwave energy, there will be a balance between saturating vapor and the predried piece humidity, with piece parts or pieces which are drier seeking to get humidity back from those which are less dry. Simultaneously, as the volume provided for saturating vapor is limited, excess vapor can only be eliminated'by condensing on surfaces which are kept cold inside the secondary cavity 22, i.e. such as metallic walls, plate 18 and particularly plaster mold free surfaces. The phenomena are somewhat more complex since, on the one hand, the secondary cavity 22 is completely water vapor tight, part of vapor leaking into the rest of the main cavity 22, and, on the other hand, some condensation occurs before the secondary cavity is full of saturating vapor at the temperature of 55C. However,

when the operation is quick enough, vapor diffusion is very low and the processing is mainly determined by the size of the secondary cavity 22 and by the temperature of walls and molds which may be assumed to be equal to 25 C.

Condensation speed is more rapid when the temperature difference between the predried pieces and the metallic walls, plate 18 and free surface of plaster molds is higher. Thus it may be of interest to increase the paste temperature, for example, up to 70C which corresponds to the safety limit. In these conditions, as shown by the curves of the FIG. 3, water mass evacuated in the secondary cavity 22 is g when the saturating vapor condition has been reached. But this result is obtained by applying a little more microwave energy and, at least during the first phase of the operation, a substantially higher microwave power than that used above.

As a result, the microwave power provided by the sources may not be constant during the whole operation. There may be two operation phases: one at high power while the piece temperature rises to the selected temperature and the second one at reduced power when the evacuated vapor has become saturated.

Passing from one power level to the other one may be made easily by manually or automatically controlling power supplies 13 of the magnetron from the control box 14 or by switching off one or several magnetrons during the second phase.

It may be noted that the most favorable operating condition is the one which at a predetermined microwave power provides:

during the first phase, a rapid temperature rise of the paste up to a temperature t, lower than the safety temperature and moderated humidity evacuation below the saturating vapor form;

during the second phase, where the paste is contacting saturating vapor, the major evacuation of the humidity.

In addition, it is possible to establish a general rule which roughly gives the required microwave energy and its distribution between the two main phases of the predrying operation when the various parameters are known. These include the volume V of the secondary cavity 22, the water-quantity A to be evacuated, the mass B of water of specific heat b included in the paste,

the mass D and the specific heat d of solid materials of the paste. Further data that is used includes:

saturating vapor temperature I, and mass (volume) M S of such vapor, which is directly dependent on t (see curves of the FIG. 3);

water vaporization heat I which is substantially constant;

piece initial temperature t which is assumed to be atmospheric temperature; Joule constant for converting energy to heat:

1 calory =Jjoules (J= 4.2).

The microwave energies E and E, respectively provided during the first and the second phase of the operation are: I

If the microwave sources are providing constant power during the whole of the two phases with the same energy density dissipated in the handled elements, then E E,. This results in a relation between t,

and M and the quantities A, B, D and V, i.e.

(bB+dD)(T,-T,,)+2IM,V=IA (3) From the formula (3), reducing V will lead to increasing t, and, thus, M,.

With the quantities used in the above mentioned example, i.e.

A=0.2 kg

B= 1.2 kg

second phase and lastly the microwave energy required for predrying 300 plates under the same conditions.

TABLE 1 V (m) (C) (2) VM. (1;) A VM. (3) (kWh) 1.8 (max.) 37.5 50 5 1.2 42.5 60 72 128 5.3 1 45 70 70 130 5.6 0.8 47.5 77 62 138 5.7 0.6 52.5 90 54 146 6.1 0.4 57.2 115 46 154 6.5 0.3 65 160 1 4a 152 7 0.25 70 200 50 7.3

The examination of this table shows that the temperature which appears most favorable is about 55C, corresponding to a volume V of 0.4 m i.e. the plate 18 is located at about 33 cm from the metallic belt 9.

The energy necessary when the plate 18 is removed (V 1.8 m may be further reduced, but the temperature of 37C is so low that the vapor condensation speed in the second phase no longer permits predrying of a plate batch in the predetermined time.

At the other limit, at the temperature of 70C, the volume V is too little to mount the plate 18 at a suitable height.

In practice, some testing using successive approximations and considering the relation (3) can determine the best volume V for the secondary cavity and microwave power to be applied when the plate flow per hour is known i.e. water flow to be evacuated as well as the temperature not to be exceeded.

The position of the plate 18 on the brackets 19 is determined at the beginning of each sequence of operation depending upon the characteristics of the pieces to be predried.

It is also convenient to have a plate which can be moved from outside the enclosure even during the predrying operations. In this case, the volume of the secondary cavity 21 can be modified, depending on the time or the temperature reached in the paste, and the position of the plate 18 can be controlled according to a program inserted in the programmed control box 14.

FIG. 4 shows a very simplified embodiment of mobile supports for the plate 18 which permit the plate to be moved easily, even during the piece predrying operation, but which avoids microwave energy leakage outside of the main cavity 21.

The ceiling 15-1 of the main cavity 15 is pierced by a number of circular holes arranged along a loop, and from which metallic cylindric tubes 24 project from the main cavity and are welded at the bases to 15-1. Each tube 24 has a length L and an internal radius r. A rod or a rope 25 of dielectric material slides in each tube, the dielectric material having low losses and a low permittivity K which may be different of the permittivity K of the plate 18.

The lowest end of each rod or rope 25 is linked to the plate 18 by any suitable means which does not use metallic material.

The rods and ropes are linked together at their upper ends by any means, such as a pulley in the case of ropes, which may control the movement through the tubes 24 and, consequently, the movement of plate 18 in cavity 15.

Each tube 24 containing a material of dielectric constant K acts as an attenuator wave guide of electric length L m and diameter 2r V K, and is a cut off device for any frequency having a wave length A which is longer than )Q/= 3.4 r VT:

The cut off wave guide introduces an attenuation a which is given by the following formula:

2 1/2 (1- 3) decibels 1/2 decibels well as for fields of wave lengths corresponding to the second and third harmonics, whose radiation outside the microwave cavity should be avoided, the parenthetic term is very close to l and the attenuation is a 55 (L/3.4r) decibels.

With L 4 cm ,.the attenuation for each tube 24 is as high as 300 decibels.

While the principles of the present invention have been described in relation to a specific embodiment, it will be clearly understood that this has only been made by way of example and does not limit the scope of the invention.

What is claimed is:

1. Microwave heating apparatus for drying wet molded articles of ceramic paste comprising:

a main enclosure having metallic walls;

a microwave source coupling microwave energy into said enclosure to heat said articles;

a secondary enclosure within said main enclosure including a wall of material transparent to microwaves, said articles being positioned within said secondary enclosure and said microwave source being disposed on said metallic walls of said main enclosure outside of said secondary enclosure, said microwave transparent wall being of sufficient tightness with respect to said main enclosure to prevent leakage of water vapor into said main enclosure from the heated articles, said main enclosure having vertical side walls and a horizontal top wall, and microwave coupling circuits on the top wall; means for supporting said articles horizontally at the bottom of said main enclosure, said microwave transparent wall including a horizontally disposed thin plate of dielectric material of low permittivity and low losses separating said main enclosure from 'said secondary enclosure, said secondary enclosure being in the lower part of said main enclosure;

and control means for applying said microwave energy for a predetermined time sufficient to raise the temperature of said articles from an initial value corresponding to room temperature to a value for evaporating a predetermined quantity of water from said articles within said secondary enclosure, the volume of said secondary enclosure being first filled with saturating vapor at said evaporating temperature, the mass of saturating vapor being substantially less than said predetermined water quantity, and thereafter maintaining evaporation at said evaporating temperature to condense excess water vapor on the colder portions within said second enclosure, the sum of the vapor mass and of the condensed water mass being equal to said predetermined water quantity.

2. Apparatus according to claim 1, including means for raising and lowering the position of said thin plate, said means including tubular sections projecting through said top wall of said main enclosure, the diameters of said sections being very short with respect to the wavelength of said microwaves.

3. Apparatus according to claim 1, wherein a higher microwave power is applied by said source in a first phase to reach said evaporating temperature than in a second phase after reaching a saturated vapor condition. 

1. Microwave heating apparatus for drying wet molded articles of ceramic paste comprising: a main enclosure having metallic walls; a microwave source coupling microwave energy into said enclosure to heat said articles; a secondary enclosure within said main enclosure including a wall of material transparent to microwaves, said articles being positioned within said secondary enclosure and said microwave source being disposed on said metallic walls of said main enclosure outside of said secondary enclosure, saiD microwave transparent wall being of sufficient tightness with respect to said main enclosure to prevent leakage of water vapor into said main enclosure from the heated articles, said main enclosure having vertical side walls and a horizontal top wall, and microwave coupling circuits on the top wall; means for supporting said articles horizontally at the bottom of said main enclosure, said microwave transparent wall including a horizontally disposed thin plate of dielectric material of low permittivity and low losses separating said main enclosure from said secondary enclosure, said secondary enclosure being in the lower part of said main enclosure; and control means for applying said microwave energy for a predetermined time sufficient to raise the temperature of said articles from an initial value corresponding to room temperature to a value for evaporating a predetermined quantity of water from said articles within said secondary enclosure, the volume of said secondary enclosure being first filled with saturating vapor at said evaporating temperature, the mass of saturating vapor being substantially less than said predetermined water quantity, and thereafter maintaining evaporation at said evaporating temperature to condense excess water vapor on the colder portions within said second enclosure, the sum of the vapor mass and of the condensed water mass being equal to said predetermined water quantity.
 2. Apparatus according to claim 1, including means for raising and lowering the position of said thin plate, said means including tubular sections projecting through said top wall of said main enclosure, the diameters of said sections being very short with respect to the wavelength of said microwaves.
 3. Apparatus according to claim 1, wherein a higher microwave power is applied by said source in a first phase to reach said evaporating temperature than in a second phase after reaching a saturated vapor condition. 